wet crawlspace

nicktJanuary 26, 2010


We built a 17 X 21 ft addition about 18 months ago. It has a 16 in deep poured concrete footer with brick and block foundation. There are vents on 3 sides to code, 6 total. The inside of the dirt crawlspace is probably a bit below grade because of the excavation and the lot has a very gradual slope from back to front. We have gutters on the addition that drain all the way out to the street through a long buried black tube. Whenever it rains hard and long, water still pools in the dirt crawlspace and stays around for a week or so. After particularly heavy rain, you can see wetness climbing up the brick and block. In between rain the floor of the crawlspace remains pretty damp.

I am fairly certain that the water is not from roof/gutters. It is hard to believe that with ALL the roof runoff carried away that so much water still moves into the crawl. Looking at the inside of the crawl, it seems like the water is just kind of seeping through the foundation (or maybe under the foundation) and pooling. I know putting down 6 mil poly and installing a sump pump would help remove the pooling water and protect the floor system from dampness, but before doing that, I would like to know if there is something to be done to prevent so much water from entering the first place. What are my options here?

Any advice would be greatly appreciated.

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Water travels in soils, especially when disturbed, in your case, with the excavation. You might consider addressing it from the outside perimeter by installing a french drain system and installing a waterproof membrane on the crawlspace walls working in conjunction with the french drain. Depending on layout, you could tie your downspouts into the french drain system as well. This is all going to be pricy and means re-excavating around the perimeter to the proper depth for the install.

    Bookmark   January 27, 2010 at 10:31AM
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Thanks sierraeast. A couple follow-up queries...

1. I am not concerned about the movement of water through the footer/foundation compromising the integrity of the structure (for several reasons I will not get into here) So,will the amount of water traveling into the crawl slow over time as the site returns to a more natural state? Meanwhile, I can put in a waterproof membrane to protect the flooring system from dampness.

2. If I decide to go the french drain route, I would only need to excavate 2 sides (one side is attached to the rest of the house and the front side has positive drainage away from house). How would I go about figuring out how deep the trenches should be and how far away from the foundation?

Thanks again!

    Bookmark   January 27, 2010 at 1:07PM
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Typically the membrane is used in conjunction with the french drain that is installed typically at or below the top of the footer. Water/moisture wicks down the membrane, into the french drain system, and out away from the house. Since the front of the house has positive drainage, you might be able to direct the drains to that point carrying it out and away. If you are going to dig down to membrane the crawlspace walls, you might as well go deeper to install the french drains as well. The trench needs to be deep enough to get a bed of gravel in before the perforated pipe is installed, then typically more gravel followed by a cloth membrane over that that keeps soils from getting into the perforated pipe. Here's an article that might help. You should also get consultations from reputable contractors in your area as they know your areas soils type.

Here is a link that might be useful: french drain install

    Bookmark   January 27, 2010 at 9:52PM
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Is the backfill around the outside of the foundation compacted and sloped away from the structure? If not, it could be acting as a funnel that is directing rainwater towards the foundation, rather than away from it.
Also, I'd check to ensure that the downspout drain tube is intact and not clogged.

    Bookmark   January 28, 2010 at 11:06AM
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